This invention generally concerns document scanners, and is specifically directed toward a scanner for producing document images having a selectable range of image types and resolutions at reduced processing bandwidths.
Optical scanners for scanning and recording optical data present on documents are well known in the art. Such optical scanners typically include a scanning station having an electronic imaging camera, a light source, and a platen formed from glass or transparent plastic for maintaining a document in a flat position during relative movement between the line-of-sight of the imaging camera and the document. For scanners limited to producing grayscale or bi-tonal images, the camera typically includes only a single linear image sensor, such as a charge-coupled device (CCD) having a resolution of anywhere between 100 to 400 pixels per inch (at the document), depending upon the desired resolution of the final image. Cameras for scanners capable of producing color images include three linear CCD imaging sensors for generating red, green, and blue image data, respectively. In operation, the documents are fed across the line-of-sight of the imaging camera, while the CCD imaging element or elements generate image data, which is temporarily stored in a buffer memory after analog to digital conversion. The stored bits of image data are then transmitted to an image processing circuit where the data is calibrated, corrected so that the resulting output mimics the physiological response of the human eye, and appropriately rotated, cropped, and scaled prior to being arranged in a form which will generate the desired image in a printer or on a video monitor.
While such prior art document scanners work well for their intended purpose, the inventors have observed several shortcomings which limit their productivity. For example, the operational speed of conventional color document scanners is limited by the high data rate associated with processing the image data generated by the trilinear CCD imaging elements in combination with the high power lighting required to illuminate the documents to the level necessary for the CCD imaging sensors to rapidly sense and transmit the color data. Of course, more powerful processors and higher intensity lighting could be used to increase the data processing rate and hence the production speed of the device. However, such a solution would substantially increase the overall cost of the scanner, as well as its power requirements. Scanners designed to operate exclusively in grayscale or bi-tonally require substantially less bandwidth for operation. They also do not require the intensity of illumination that a trilinear, color filtered CCD imaging sensor requires. Consequently, such scanners are capable of producing images of the same resolution as those produced by color scanners in a fraction of the time and with substantially less power. However, the resulting images cannot provide any information with respect to color. In certain applications, the lack of such a color capacity severely limits the usefulness of the scanner. Finally, while the documents being scanned in a single scanning run often have differing image requirements (i.e., many may be scanned in grayscale or bi-tonally at either high or low resolution, while others require color scanning) there presently exists no scanner capable of providing the desired range and choice of image types and resolutions in a manner which makes efficient use of bandwidth. For example, even though it is possible to generate grayscale images from a color scanner, the processing circuit of such color scanners still requires a higher degree of bandwidth than if a grayscale scanner were used. Thus production speed is not maximized. Also, it is difficult to generate accurate bi-tonal images from a color-filtered, trilinear CCD imaging array.
Clearly, what is needed is a document scanner, which is capable of providing color image information at production speeds substantially higher than those associated with conventional color document scanners without the need for expensive, high capacity processors or higher-powered lighting. Ideally, such a scanner should be capable of providing images of either high or low resolution which may be color, grayscale, or bi-tonal in order to meet the specific needs of a broad array of end users. Finally, it would be desirable if the image processor of such a scanner used only the amount of bandwidth necessary to produce the particular type of document image selected by the end user so that production speed at all selections is maximized.
Generally speaking, the invention is a document scanner that overcomes the limitations associated with the prior art by providing either color, grayscale or bi-tonal images having a selectable range of resolutions with a minimum amount of bandwidth. To this end, the document scanner comprises an imaging camera including a grayscale electronic imaging element for generating high-resolution grayscale images, and a plurality of color imaging elements for generating low-resolution color images. The scanner further comprises an image processing circuit for efficiently processing the data generated by the imaging elements into a selected one of color, grayscale, or bi-tonal document images in either high resolution or low resolution.
The grayscale imaging element is preferably a linear CCD image sensor of high resolution having between about 300 to 600 pixels per inch relative to the document. Each of the color imaging elements is also preferably a linear CCD having a lower resolution capacity of between about 100 to 300 pixels per inch. Preferably, all four of the linear CCD image sensors are arranged mutually parallel, wherein the center portion of the pixels of the grayscale sensor are aligned with a side edge of the pixels of each of the color sensors in order to minimize the generation of unwanted artifacts in the final image.
A driver circuit is connected to each of the CCD image sensors for conveying image data from the sensors to buffer amplifiers. Analog to digital circuits are provided for digitizing the image data from the buffer amplifiers and for conveying the digitized data into an image processor.
Depending upon the particular type of document image selected by the end user, the processor circuit proceeds to generate either a color, grayscale, or bi-tonal image that is either high or low resolution. If high resolution color is selected, the processor generates a composite of a high-resolution grayscale image in combination with the color images generated by the lower resolution color sensors. Because the human eye is more sensitive to the high-resolution grayscale image in the resulting composite color image, the resulting composite image provides a color image of high subjective quality with far less processing than would be required by a conventional color scanner camera utilizing three high-resolution CCD image sensors. Moreover, when color image information is not required or desired in the end product, the image processor is capable of providing either high or low resolution grayscale images or bi-tonal images with the fraction of the processing required for color images. Additionally, the processor can generate any combination of color, grayscale, and bi-tonal images. In all cases, production speed is maximized by the selectability of the specific type of document image needed by the end user and the optimum use of processing resources to execute the particular type of document images requested.